type Assignment = { kind: "assign"; key: string; value: string };
type Flag = { kind: "flag"; key: string };
type Token = Assignment | Flag | { kind: "comment"; text: string };
const tokenize = (line: string): Token => {Use utility conditional types to parse and narrow a small input grammar.
interface LoggerPlugin { kind: "logger"; log(message: string): void }
interface CachePlugin { kind: "cache"; get(key: string): unknown }
export type AppPlugin = LoggerPlugin | CachePlugin;
Narrow a plugin union by checking which method it has, instead of switching on a kind tag, so adding a capability-only plugin needs no tag.
export function assertIsString(value: unknown, name: string): asserts value is string {
if (typeof value !== "string") {
throw new TypeError(`${name} must be a string, got ${typeof value}`);
}
}Narrow a value by throwing instead of branching, so the rest of the function can assume the checked type without an extra if.